The structure of nanofibers from polymers is expected to bring about unique mechanical, electrical, and optical properties, which are different from bulk materials similarly to the nanoscale versions of many metals or semiconductors. Polymer nanofibers should be considered as essential “building blocks” of the nanoscale toolset along with a large variety of inorganic “building blocks” well-known in materials science, which include nanoparticles, nanowires, carbon nanotubes (CNTs), graphene, and clay nanosheets. Liquid dispersions of polymer nanofibrils analogous to those obtained for all the other inorganic nanoscale components (INCs) would be most useful for the functional design of nanocomposites and metamaterials. Variable permutations of organic components in the forms of molecular solutions or nanofiber dispersions and INCs will make possible property sets which are currently considered to be difficult or impossible to obtain. However, the choice of polymer nanofiber dispersions is limited at best or not known for most polymers. Compared to dispersions of INCs this is virtually a virgin territory for both chemical and materials research. Polymeric nanofibers are typically produced by electrospinning, drawing, template synthesis, phase separation, and self-assembly. Electrospinning is probably the most widely used method for generating polymeric nanofibers with controlled diameters (from several nanometers to micrometers). This method is very useful for generation of solid nanofiber mats from many different polymers with typical diameters from tens of nanometers to tens of micrometers, but not for very small fibers comparable with CNT diameters or nanofiber dispersions. One can also obtain polymeric nanofibers by template synthesis, for instance in solid oxide matrices followed by their dissolution. This approach can be modified for the preparation of multi-segmented polymer metal nanorods of interest for electronic devices, but it is limited by only a few available templates, relatively large diameters exceeding 50 nm, and is restricted to the interfacial localization of the nanofibers. The yield of the nanorods made from templates is also very small. The small amounts of material that can be produced and the inability to disperse nanofibers in liquid media are also characteristic for many nanofibers made by interfacial reactions. Many examples can be seen for electropolymerized nanofibers of conducting polymers, such as polyaniline nanowires, which typically produce fibrils with diameters of 50-70 nm. There are also techniques for making polymeric nanofibers by phase separation, 6 or interfacial polymerization. They can be convenient for preparation of solid porous materials, but are difficult to use as a general source of nanoscale organic components for materials design. The nanofibers made in this way often exceed 30 nm in diameter. An interesting technique that avoids the interfacial restrictions typical for the methods discussed above is the method of self-assembly of purposely designed organic surfactants. This method can be compared to the assembly of inorganic nanoparticles, and leads to smaller fibers with diameters of less than 10 nm. Some of the self-assembled organic fibers can be made in a dispersed state instead of the more common gels. Although the synthesis of the corresponding surfactants requiring a combination of hydrophilic and hydrophobic blocks is quite complex, these nanofibers are quite interesting for biomedical applications as cell-growth matrices. At the same time, they represent a substantial departure from the idea of polymeric nanofibers with predominantly axial orientation of molecular strands as obtained, for instance, by electrospinning. The radial orientation of the amphiphilic units and small area of interactions between them are not necessarily optimal for mechanical, electrical, and optical properties.
One can see that there are no known examples of dispersions of synthetic polymer nanofibers in size matching those of CNTs, nanowires, nanoparticles, and other INCs. Finding methods for their preparation and utilization of a broader nanotechnology toolbox would be essential for further development of nanomaterials.